Amorphous elastic-plastic deformation

Let us consider two limiting cases of the adduced in Fig. 4.15 dependence at = 0 and 1.0, both at d = 3. In the first case (d = 2) the value dU = 0 or, as it follows from dU definition (the Eq. (4.31)), dW = dQ and polymer possesses an ideal elastic-plastic deformation. Within the frameworks of the fractal analysis d =2 means, that (p, = 1.0, that is, amorphous glassy polymer structure represents itself one gigantic cluster. However, as it has been shown above, the condition d =2 achievement for polymers is impossible in virtue of entropic tightness of chains, joining clusters, and therefore, d > 2 for real amorphous glassy polymers. This explains the experimental observation for the indicated polymers dU 0 or dW dQ [57], At Vg, =... 

In comparing the correlation sought between MH and E one should emphasize the following while the plastic deformation of lamellae at larger strains when measuring MH depends primarily on crystal thickness and perfection in case of the elastic modulus the major role is played by the amorphous layer reinforced by tie molecules, which is elastically deformed at small strains. Figure 17 illustrates de... 

TWO SEPARATE ALTERED or damaged layers classically have been recognized on metal surfaces formed by cutting- or polishing-type processes namely, an amorphous-like "Beilby" layer and a plastically deformed layer. Modern work indicates that the Beiiby layer is not, in fact, formed by the common important methods of surface preparation but that a deformed layer always is. The detailed structure of this layer is reviewed. Some consideration is also given to residual elastic stresses, surface topography, and embedded abrasive. 

An experimental relationship between the microhardness and elastic modulus (E) has been found for various PE materials with different crystallinity values (Flores et al.., 2000). It is important to realize that microhardness - the plastic deformation of crystals at high strains - primarily depends on the average thickness and perfection of the nanocrystals, whereas in the case of the modulus, the elastic response at low strains is dictated by the cooperative effects of both microphases, the crystalline lamellae and the amorphous layer reinforced by tie molecules. The... 

There have been many efforts for combining the atomistic and continuum levels, as mentioned in Sect. 1. Recently, Santos et al. [11] proposed an atomistic-continuum model. In this model, the three-dimensional system is composed of a matrix, described as a continuum and an inclusion, embedded in the continuum, where the inclusion is described by an atomistic model. The model is validated for homogeneous materials (an fee argon crystal and an amorphous polymer). Yang et al. [96] have applied the atomistic-continuum model to the plastic deformation of Bisphenol-A polycarbonate where an inclusion deforms plastically in an elastic medium under uniaxial extension and pure shear. Here the atomistic-continuum model is validated for a heterogeneous material and elastic constant of semi crystalline poly( trimethylene terephthalate) (PTT) is predicted. 

The four-parameter model provides a crude quahtative representation of the phenomena generally observed with viscoelastie materials instantaneous elastie strain, retarded elastic strain, viscous flow, instantaneous elastie reeovery, retarded elastie reeovery, and plastic deformation (permanent set). Also, the model parameters ean be assoeiated with various molecular mechanisms responsible for the viscoelastic behavior of linear amorphous polymers under creep conditions. The analogies to the moleeular mechanism can be made as follows. 

Demkowicz, M. J. and Argon, A. S. (2005b) Autocatalytic avalanches of unit in elastic shearing events are the mechanism of plastic deformation in amorphous silicon, Phys. Rev. B, 72, 245205 (1-16). 

These ductile features of these glasses may be observed microscopically in Fig. 3.18a and b, respectively. However, the stress—strain curves of these Zr-based amorphous glasses did not display appreciable macroscopic plastic deformation prior to catastrophic fracture, rather they mainly deformed elastically, followed by catastrophic failure along their shear bands. Examination of the fracture regions... 

As previously stated, ceramics are characterized either by the ionic or covalent bonding of their constituents and, consequently, with some exceptions, they exhibit brittle behavior. Also note that the field of ceramics covers a broad range of structures, from completely crystalline to amorphous (mostly glassy structures). Therefore, the main deformation at ambient temperatures is elastic (tending to brittleness) only at elevated temperatures may one speak about plastic deformation, since most ceramics show ductility. Clearly, the temperature level is a... 

During elongation of a semicrystalline polymer several different processes, sudi as elastic deformation of the original spherulitic superstructure, tran ormation of a spherulitic into a fibrillar structure, plastic deformation of microfibrils by sliiqiage processes and elongation of molecular chains in the amorphous regions may occur simultaneously, successively or partly superimposed One of the advanta s... 

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